1. Field of the Invention
The present invention is related to a purification method of a resin solution, and in particular to a purification method of a polyamic acid (PAA) resin and a polyimide resin.
2. Description of Related Art
Performance of electronic material products at the application end often determines the quality and performance of devices, so that compared with materials of general use, electronic materials having greater threshold requirements in purity and quality. In order to satisfy requirements of the end products, in synthesis of electronic material, polymerization of various kinds of starting material or formulae having different materials added is used, so that if purity monitoring is performed on the most initial starting materials, variations in qualities of the electronic materials are reduced.
Taking polyimide (PI) and polyamic acid (PAA) for example, they are widely used as electronic materials. PI is used in electronic products such as liquid crystal displays (LCD), integrated circuits (IC), and flexible copper clad laminates (FCCL). For applications in different products, there are different requirements on polyimide and polyamic acid. In addition to the different requirements, however, there are common requirements for various applications in both the metal ion contents and water contents in polyimide and polyamic acid, since these two factors directly affect the performances of the end products.
For example, when polyimide or polyamic acid is used in orientation films, it is required that the metal ion content therein is low. Nonetheless, the main functional groups in the compositions of resin solutions are amino groups, imide groups, amide groups, and/or acidic groups, which have chelating forces on the metal ions. In addition, the solvents (such as 1-methyl-2-pyrrolidone, cyclobutyrolactone, butyl carbitol (BC), ethyl carbonate (EC), and dimethylacetamide (DMAc)) which are used also have chelating abilities. Hence, after the resin solution is used, the metal ion impurities contained therein are often excessive, which calls for further purification. Moreover, in the application of orientation films, polyimide or polyamic acid requires low water content, in consideration of its effect on planarity and uniformity of the film surface during coating. Hence if the water content increases during production or after use, the water content needs to be reduced.
In current purification of polyimide or polyamic acid, precipitation is the mainly used method. This method is to remove polymers or monomers in the solution which have smaller molecular weights, thereby narrowing the distribution of molecular weights. During the processes, the metal ions are simultaneously washed out, so as to achieve purification effects.
This precipitation method, however, is complex in processes, and a great amount of the solvents is consumed therein for industrial use, which leads to further problems of subsequent waste solution. In light of the above, if a purification method with simplified processes and reduced solvent consumption is offered, there would be great benefit in both practical applications and environmental protection.